With remarkable advances in the performance of computers and other information equipment, the volume of information that is handled by users has been constantly increasing and is now measured in gigabytes as a unit. Under these circumstances, there exists an ever-growing demand for semiconductor devices such as information storage/reproduce equipment and memories that are capable of recording at even higher densities.
To achieve higher recording densities, technologies for even finer microfabrication are required. Conventional photolithography which uses the exposure process is capable of microfabrication over a large area in one step; however, since its resolution is not finer than the wavelength of light, conventional photolithography is inevitably unsuitable for creating fine structures smaller than the wavelength of light (say, 100 nm and less). Technologies currently available for processing finer structures than the wavelength of light include exposure using electron beams, exposure using X-rays, and exposure using ion beams. However, pattern formation with an electron beam lithographic apparatus differs from patterning by one-shot exposure using such light sources as i-line and an excimer laser in that the more patterns that need be written with electron beams, the longer the time that is required for writing (exposure). Therefore, as the recording density increases, the time it takes to form a fine pattern is extended to cause a marked drop in throughput. With a view to forming patterns at a faster speed by the e-beam lithographic equipment, the development of a method for one-shot irradiation of geometric figures is underway in which combinations of variously shaped masks are subjected to one-shot exposure to electron beams; however, the e-beam lithographic apparatus that uses the method for one-shot irradiation of geometric figures is not only bulky but it also needs an additional mechanism for controlling the positions of masks to an even higher precision; this increases the cost of the lithographic apparatus, eventually leading to a higher cost for manufacturing media.
Printing-based approaches have been proposed as an alternative to the conventional exposure technologies for creating fine structures smaller than the wavelength of light. See, for example, the article titled “Imprint of sub-25 nm vias and trenches in polymers” that is carried in Non-Patent Document 1. Nanoimprint lithography (NIL) is a technique in which a pattern of a predetermined fine structure is formed on a master by exposure to electron beams or using some other methods of creating finer structures than the wavelength of light and the master is urged under pressure against a resist-coated transfer substrate so that the fine structured pattern is transferred to the resist layer on the transfer substrate. As long as the master is available, there is no particular need to employ an expensive exposure unit but an apparatus in the class of ordinary printing presses will suffice to produce replicas in large quantities; hence, in comparison with the conventional methods such as exposure to electron beams, there is achieved a marked improvement in throughput whereas the manufacturing cost is significantly reduced.
When a thermoplastic resin is used as a resist material in the nanoimprint lithographic (NIL) technology, transfer is performed with the thermoplastic resin being heated under pressure to a temperature near its glass transition temperature (Tg) or higher. This approach is called a heat transfer technique and described in Non-Patent Document 2. The heat transfer technique has the advantage of permitting the use of general-purpose, thermoplastic resins. If a photosensitive resin is used as a resist in the NIL technology, a photocurable resin that hardens upon exposure to light such as UV radiation is chosen as the resin to which the original fine pattern is transferred. This approach is called an optical transfer technique and described in Non-Patent Document 3.
In the nanoimprint processing technology using the optical transfer technique, a special photocurable resin must be used but, on the other hand, it has the advantage of reducing the dimensional errors in finished products due to the thermal expansion of transfer printing plates or printing media. Other advantages that are related to the apparatus include elimination of the need for equipping it with a heating mechanism and providing accessories such as for performing temperature elevation, temperature control, and cooling. There is a further advantage concerning the nanoimprint apparatus taken as a whole and that is elimination of the need for design considerations against thermal distortions, such as heat insulation.
An example of nanoimprint apparatuses based on the optical transfer technique is described in Non-Patent Document 3, ibid. This apparatus is so designed that a quartz or sapphire mold (master) capable of transmitting UV light is urged against a photocurable resin coated transfer substrate and irradiated with UV light from above. However, the patterned structure on the rigid quartz or sapphire mold is known to be easily damaged if the mold is pressed into contact with a rigid transfer substrate.
In order to realize uniform and flawless transfer, the fine structure formed in a surface of the stamper need be brought into intimate contact with a surface of the transfer substrate. However, if the transfer substrate itself has a warpage or if a foreign object gets seated between the rigid mold and the transfer substrate or in the presence of any irregular protrusions on the surface of the transfer substrate, a gap or gaps may sometimes occur between the rigid mold and the transfer substrate to inhibit them from having intimate contact with each other. As a result, when a pattern is formed in the photocurable resin, the thickness of the base layer is greater than it should be by an amount that corresponds to the created gap or gaps. The thick base layer cannot be etched away, which eventually becomes a major cause of a poorly etched final product.
With a view to solving these problems, Patent Document 1 proposed that a polymer stamp, or a polymeric material to which the pattern on a rigid mold has been transferred, should be substituted for the rigid mold as a secondary replica. Since the polymer stamp is soft and resilient, it can be pressed with great force into contact with a rigid transfer substrate, with only a small likelihood for the occurrence of unwanted accidents such as nicking of the pattern in the stamp; what is more, the entire surface of the stamp except in the areas where the protrusions or foreign objects occur makes intimate contact with the transfer substrate, so the thickness of the base film becomes thin enough to permit its removal by etching. In addition, given the rigid mold which serves as a master, as many polymer stamps as are required can be produced, so the polymer stamps themselves can be manufactured at such a low cost that a single stamper may be used a plurality of times to cut the print cost or, alternatively, it may be discarded after being used once or several times.
FIG. 13 is an illustration that shows diagrammatically an imprint process that is carried out with a nanoimprint stamper using an example of the polymer stamp proposed in Patent Document 1, ibid. In FIG. 13, the numeral 100 designates a stamper with a hard backup according to the conventional technique. A backup plate 102 is a thick-walled, transparent glass plate. A transparent polymer stamp 106 having a patterned layer 104 on its surface is held on a transparent stamper base 110 with a transparent multi-layered resilient layer 108 being interposed. A transfer substrate 112 is placed on the topside of a substrate platform 114. For the sake of clarity, the warpage of the transfer substrate 112 and the protrusions 116 on the substrate surface are exaggerated.
The polymer stamp 106 is typically formed of a transparent polyester resin with a thickness of 0.5 to 5 μm and a Young's modulus of 2 to 3 GPa. The stamper base 110 is a transparent glass or plastic material with a thickness of about 2 mm. The multi-layered resilient layer 108 is typically formed of a polyurethane rubber, a silicone rubber or an acrylic rubber and the number of layers that compose it generally ranges from about 4 to 6. The presence of this multi-layered resilient layer 108 made of the multi-layered resilient body has enabled the polymer stamp 106 to conform to the warpage of the substrate 112 or any protrusions or invading foreign objects 116 on the substrate surface.
However, in the case where the stamper 100 is designed to have a resilient structure so that it conforms to the warpage of the substrate 112 or protrusions or foreign objects 116 on the substrate surface, if the stamper 100 is rendered soft enough to conform to protrusions or foreign objects 116 of shorter wavelength, “waviness” of longer wavelength is generated and the stamper 100 becomes less capable of conforming to the warpage of the substrate; on the other hand, if the stamper's ability to conform to the warpage of the substrate is enhanced, it becomes less capable of conforming to the protrusions or foreign objects; thus, it has been difficult in the prior art to ensure that the warpage of the substrate and the protrusions or foreign objects on its surface that differ greatly in the wavelength of their shape variation can be coped with simultaneously.